JPH01103071A - Finder optical system for single lens reflex camera - Google Patents

Abstract

PURPOSE:To attain accurate focusing without any range finding error by adopting second-order image forming type finder system in a still video camera of single lens reflex type and observing an image of a focus plate arranged on the secondary image face. CONSTITUTION:A primary image face 13a has no magnification, the primary image face 13a is formed on a focus plate 21 by using a image forming lens system 17 through a field lens 14 making a radiation pupil of an image pickup lens conjugate with an incident pupil of the lens 17, the similar optical performance to the image pickup face is provided to the focus plate 21, the formed secondary image is observed by an eye contact lens 23 to apply focusing. Then an image face flat lens 20 whose concaved face is directed to the secondary image forming lens is arranged near the image forming lens of the focus plate 21 to apply accurate focusing without mis-range finding.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention uses an imaging member such as an imaging tube or a COD,
The present invention relates to a finder optical system suitable for a single-lens reflex electronic camera that electrically processes still images.

[Prior art]

Conventional single-chip color video cameras require a striped or mosaic color filter to be placed in close contact with the image sensor in order to obtain color signals, which makes the pixels of the image sensor itself smaller. Even if the number is large, the resolution actually decreases depending on the pitch of the color filter.

First, FIG. 3 shows an example of the optical system of a single-panel reflex electronic camera.

101 is a photographing lens, 102 is a movable mirror (quick return mirror), 103 is a low-pass filter, 1o4
is the shutter unit, 1o5 is the cover glass, 106
is an image sensor, and 107 is a package of the image sensor. Photographing light from the photographing lens 101 is filtered through a low-pass filter 1.
03, image sensor 10 through a stripe filter etc.
The image is formed on 6. Reference numeral 108 denotes an optical correction glass, which is arranged to make the image observed through the finder optically equivalent to the image on the image pickup device formed by the light flux passing through the low-pass filter. 109 is a focusing plate, 110° 111 is a prism that allows the pupil 113 to observe an erect normal image, and 112 is an eyepiece lens.

In a single-panel single-lens reflex camera, the glass on the image sensor 105 side of the movable mirror 102 is relatively thin and consists only of the optical low-pass filter 103, so it can be placed in front of the focus plate 107 of the finder system without taking up much space. A thick optical correction glass 108 can be arranged, and the focusing plate 10
An image equivalent to the imaging surface can be formed on the surface 7.

By the way, in the case of a two-panel or three-chip camera, the resolution is improved by increasing the number of image sensors, but in addition to a low-pass filter, there is a resolving prism between the total reflection mirror that reflects the photographic light flux to the viewfinder and the imaging surface. exists. Therefore, to obtain the same performance as the photographing system, it is necessary to insert a similarly thick optical correction glass between the movable mirror and the primary imaging surface in the finder system. However,
Inserting an optical correction glass so as not to blur the photographing light flux results in a limit to the thickness of the glass that can be inserted due to mechanical constraints. Therefore, the image performance of the finder system is different from that of the imaging system, and if a focus plate is placed on the primary image forming plane of the finder and the focus is adjusted, a problem arises in that defocus actually occurs.

[Purpose of the invention]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, it is an object of the present invention to provide a finder optical system that allows the viewfinder to observe an image on the same level as the imaging surface even in a two-panel or three-panel camera.

Therefore, the present invention provides a viewfinder optical system for a single-lens reflex electronic camera that records video information based on a photographic lens and a decomposition prism that decomposes a convergent light flux emitted from the photographic lens. A secondary imaging system for re-forming an image, a focusing plate disposed near the secondary imaging plane of the secondary imaging system, and an eyepiece for observing the image on the focusing plate. This is because the secondary imaging system performs aberration correction substantially equivalent to that of the imaging surface.

〔Example〕

Hereinafter, the present invention will be explained in detail based on the drawings.

FIG. 1 is a diagram showing an embodiment of the present invention. In FIG. 1, 1 is a photographing lens, 2 is a movable mirror (quick return mirror), and 2', indicated by a chain line, is a position where the total reflection mirror 2 rotates around a fulcrum 2a as a center of rotation during photographing, and the total reflection mirror 2 retreats. Prisms 3 and 4 are for color separation or luminance signal light separation, and the boundary surface between the prisms 3 and 4 is a half mirror.

5.9 is an optical low-pass filter, 6 and 10 are shutters, 7.11 is an image sensor unit, 8.12 is a signal processing circuit that converts one image into an electrical signal, and 13 is an optical correction glass. It is shorter than the optical path length of prism 3.4. 13a is the finder primary image plane, 14
is a field lens placed near it. 15 is a reflecting prism for bending the optical path, 16 is a total reflection mirror,
17 is a re-imaging lens, and 18 and 19 are total reflection mirrors. 20 is a concave lens with an image plane flattening function, 21 is a focus plate, and 22 is a field lens.
2 is placed close to the focus plate 21. 23 is an eyepiece lens, and 24 is an observer's eye. The reflecting prism 15 and the mirror 16° 18.19 bend the optical path into a parallelogram shape to increase the optical path length so that the eyepiece 23 is placed at a predetermined position.

In the configuration shown in FIG. 1, the separating prism is composed of prisms 3 and 4 so that color signals and luminance signals can be obtained independently from separate image pickup devices 7 and 11. Further, the optical low-pass filters 5 and 9 are inserted on the side of the image sensor that obtains color signals mainly to remove false color signals, and on the side of the image sensor that obtains luminance signals mainly to prevent the occurrence of moal. Therefore, in order to give the finder system the same optical performance as the photographing system, it is necessary to provide guarantees that include a resolving prism and an optical low-pass filter.

By the way, since the resolving prisms 3 and 4 are quite thick, the movable mirror 2 is located relatively closer to the photographing lens side than in a single-panel electronic camera. In other words, the photographing light beam is spread out considerably, and the position 2' of the movable mirror at the time of exposure must be set at a fairly high position so that the photographing light beam is not eclipsed.

Therefore, the glass thickness of the prism 13 in the space up to the primary image plane 13a of the finder system is 3.4
It is considerably thinner than the glass thickness including the optical low-pass filter. Therefore, when the primary image plane focusing plate is placed and the focus is adjusted, sufficient correction cannot be made due to the difference in glass thickness, resulting in poor image performance. Different on the 5th image plane and the imaging plane,
This will result in a focus shift. Therefore, in this example,
The primary image plane 13a is a transparent surface, and the primary image plane 13a is projected onto the focus plate 21 by the re-imaging lens system 17 through a field lens 14 that makes the exit pupil of the photographing lens and the entrance pupil of the re-imaging lens 17 conjugate. The focal plate 21 has the same optical performance as the imaging surface, and the formed secondary image is observed through the eyepiece lens 23 to perform focusing.

That is, differences in spherical aberration caused by differences in the glass thickness of the imaging system can be eliminated by somewhat narrowing down the F-number light beam with the re-imaging lens 17;
As a result of the addition of the field curvature that occurs, a new problem arises in that out-of-focus occurs when focusing around the screen of the reticle 21. Therefore, focusing plate 2
The above-mentioned problem is solved by disposing an image plane flattening lens 20 having a concave surface facing the secondary imaging lens side near the re-imaging lens 1. In addition, by inserting the image plane flattening lens 20 having negative refractive power, the relationship between pupil and image formation is improved, and the observation position 24
(This problem is solved by inserting the field lens 22 near the focusing plate 21.)

Since the performance of the entire finder system may be degraded by the re-imaging lens 17, the diameter of the re-imaging lens 17 should be suppressed and the aperture of the photographing system should be substantially stopped down when observing. Practically preferable results can be obtained rather than observing with a primary image and always causing out-of-focus.

Next, numerical examples according to the present invention will be shown below.

FIG. 2 is a diagram showing the finder optical system of the present invention developed along the optical axis. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and νi are the curvature radius of the i-th lens from the object side, respectively. The refractive index and Atsbe number of glass, former Di Ni 27 ■ 1
Eye point vision
-1 Dayopter [Effect] As explained above, in a single-lens reflex type still video camera that requires a resolving prism in the photographing system, the finder system is a secondary imaging type and the focal point placed on the secondary image plane. By observing the image on the plate, accurate focusing without distance measurement deviation is possible.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a single-lens reflex electronic camera according to the present invention, FIG. 2 is a cross-sectional view of a lens developed with respect to the optical axis of the finder optical system according to the present invention, and FIG. 3 is a conventional general FIG. 2 is a sectional view of a typical electronic camera. In the figure, - 2 is a movable mirror, 3 and 4 are resolving prisms, and 17 is 2
The next imaging lens 21 is a focusing plate. gong

Claims (3)

[Claims] (1) In the finder optical system of a single-lens reflex electronic camera that records video information, the image formed by the photographic lens is Re-image 2
A single lens comprising a secondary imaging system, a focusing plate disposed near the secondary imaging plane of the secondary imaging system, and an eyepiece for observing an image on the focusing plate. Finder optical system of reflex electronic camera. (2) A finder optical system for a single-lens reflex electronic camera according to claim 1, characterized in that a negative lens is disposed on the secondary imaging system side of the focus plate, and a positive lens is disposed on the eyepiece side. (3) A finder optical system for a single-lens reflex electronic camera according to claim 1, characterized in that there is a movable mirror between the photographing lens and the prism that retreats from the photographing light beam during exposure.